Phys 115 1st Edition Lecture 22Outline of Last Lecture I. Single Slit InterferenceOutline of Current Lecture II. PhotonsIII. Quantum StatesIV. AbsorptionV. FluorescenceVI. PhosphorescenceVII. BioluminescenceCurrent Lecture- Photonso Light behaves like a wave, but also like a particleo The energy of a photon (particle of light) is related to its frequency: E = hfo h = Planck’s constant = 6.63 x 10-34 J-so More convenient: h = 4.14 x 10-15 eV-so Speed of light c = fλ→ f = c/λ→ E =hf= hc/λo hc = 1240 eV-nmo Visible light: λ = 400-800 nm o Blue light photon: E = hc/λ=1240 eV-nm/400 nm = 3.1 eVo Red light photon: E = hc/λ=1240 eV-nm/620 nm = 2.0 eVo Total energy in a light beam = (total energy per photon) x (number of photons)o UV light: λ = 300-400 nmo E ~ 3 – 4 eVo Equal to typical chemical bond energies Sunburn! o Solar spectrumo Sun emits a broad spectrum related to its temperature: blackbody spectrum- Absorptiono Atoms and molecules have discrete energy levelso Transitions between levels can result in emission of photonso Photon energy = difference in energy between levelso Atom or molecule can absorb a photon and make a transition to a higher energy stateThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Energy of absorbed photon = energy difference between initial and final energy stateo Molecules also have energy levels from vibrational and rotational stateso These states are closer together than those for different electron orbitalso Closely-spaced initial and final states → broad absorption (and emission) bandso Energy levels (thus absorption energies) depend on chemical bonding- Absorption and coloro Pigment color results from wavelengths of light not absorbed by materialo Light reflected from organism has penetrated surface and been scattered back byunderlying tissueo Some wavelengths absorbed (going in or out), those not absorbed determine coloro Red paint = quinacridone (pigment) + TiOo Absorption and coloro Red paint = quinacridone (pigment) + TiO2 + matrix- Fluorescenceo Atom or molecule in excited state (after absorption) can emit a photon: fluorescenceo Emission energy < absorption energyo “Missing” energy: vibration, rotation, reactions (e.g. photosynthesis), …o Emission occurs a few nanoseconds after absorption (or sooner)- Green fluorescent protein (GFP)o Derived from jellyfish Aequoreavictoria- Phosphorescenceo Like fluorescence, but slowero Emission milliseconds or even minutes or hours after absorptiono Downward transitions “forbidden” (highly improbable) by quantum mechanicso Phosphorescence of amino acid tryptophan in human red blood cell membraneso Study organization of proteins in cell membraneo Emission persists for several seconds- Bioluminescenceo Excited state produced by chemical reaction rather than light absorption (chemiluminescence)o Transition to ground state results in emission of lighto Fireflieso Non-biological: glow
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